NHS productivity has "almost certainly" risen in the past decade, with taxpayers getting more out of the health service for every pound spent – undermining one of the government's key arguments for its reforms, according a new paper.

In a paper published in the Lancet, Nick Black, professor of health services research at the London School of Hygiene and Tropical Medicine, said that although the health secretary, Andrew Lansley, claimed NHS productivity had fallen 15%, the opposite was almost certainly the case.

"Despite such confident statements, rather than declining, the productivity of the NHS has probably improved over the past decade," Black said. "To justify the reforms to the NHS that the Conservative party wanted to introduce, the claim of declining NHS productivity was necessary."

Black, who is an adviser to the Department of Health and is widely respected across the political spectrum, challenged the view that productivity in NHS hospitals had dropped by 1.4% a year despite the budget swelling from £60bn in 2000 to £102bn in 2010.

However, Black produces a slew of evidence that questions the analysis of the Office of National Statistics used to work out the productivity of the health service. The ONS looked at the return for taxpayers by comparing public expenditure with how much patients used the health service and what the outcomes were.

Black's work, the first of its kind, argues that the measures the ONS used do not reflect the substantial improvements in NHS care. It points out that between 2000 and 2009, such were the advances that a baby born in 2009 could expect to live three years longer than one born in 2000.

Black says far fewer people were dying in specialist procedures in the NHS. He notes declines occurred in adult critical care (2.4% a year), dialysis (3.3% a year), and coronary artery bypass surgery (4.9% a year).

Patients' experience of how they were treated also improved. There were annual relative increases in the proportion of patients treated within four hours in accident and emergency departments (2.5% a year) and in the numbers operated on within 28 days of their operation having been cancelled for non-clinical reasons (10.4% a year).

Such was the NHS's popularity that in the annual British Social Attitudes survey, 70% of respondents reported they were overall satisfied with the NHS. This was the highest figure ever recorded by the long-running survey – the lowest was 34% in 1997, at the end of the Conservatives' 18-year tenure in office.

Black accepts that pay rose for frontline staff in the NHS but says this does not mean money was wasted. "The myth is that lots of extra money went into the NHS and productivity went down. Easy to see why if you pay a surgeon 30% more and he still works the same number of hours, clearly you think productivity went down. But this neglects quality improvements which you can measure."

The paper does not put a figure on the rise in NHS productivity, and Black said this would need more work. "It's beyond the scope of this work. However I am certain NHS productivity did not drop."

The Department of Health responded with a statement from the health minister Simon Burns. He said: "We have always been clear that productivity in the NHS needs to improve and are committed to better outcomes for patients across the country.

"We are investing an extra £12.5bn in the NHS, but we want to make every penny count. We know the NHS can meet this challenge – we have already made £7bn in efficiency savings over the last 18 months as performance has improved: record low infection rates, mixed-sex wards down by over 90% and people waiting over a year reduced by half."

Andy Burnham, the shadow health secretary, said: "This analysis is hugely embarrassing for the prime minister. It demolishes an anti-NHS argument that Cameron and his ministers have repeatedly trotted out. Far from falling, NHS productivity increased in the last decade at the same time as the NHS was achieving record patient satisfaction. It is this successful NHS that, inexplicably, is being turned upside down by the Tory-led coalition.

"But, as well as destroying their arguments, this analysis exposes the prime minister's duplicity on the NHS. Professor Black explicitly criticises the Tories for propagating a myth that NHS productivity was declining to create a false justification for their health and social care bill."